Road surfaces (roads) on which vehicles drive are marked or installed with various road markings such as white lines, yellow lines and cat's eyes constituting traffic lane marking lines. Conventionally the recognition of white lines and other such traffic lane marking lines has, as taught by Patent Reference 1, been done by subjecting an image photographed or taken by image photographing means such as a CCD camera and an image processing ECU to differentiation and binarization processing to detect edges in the image and subjecting point sequences of the detected edge (white line candidate point sequences) to Hough transformation to extract approximated linear components.
Further, when, for example, a white line marked on the road surface is itself physically worn through or partially chipped away, or the white line is indistinct in the photographed image owing to low contrast such as may occur during night driving, the white line cannot be accurately recognized, and, as set out in Patent Reference 2, a technique has been proposed for, in such a case, forming a window in the photographed image, judging the degree of white line wear-off from the density of the white line candidate points in the window, and when wear-off is found, superimposing/compositing a white line candidate point sequences extracted from a photographed image taken a given time earlier on/with the white line candidate point sequences extracted from the current white line image, determining a straight line approximating the composited white line candidate point sequences, and recognizing it as the white line.
Patent Reference 1: Japanese Patent Publication No. Hei 6(1994)-42261
Patent Reference 2: Japanese Laid-Open Patent Application No. 2001-236506
Now, cases arise in which the posture or angle of the vehicle with respect to a traffic lane marking line differs between the present time point and the earlier time point, so that the photographed images deviate or shift in the horizontal direction or the angular direction of the vehicle with respect to the traffic lane marking line between the two time points, which of course means that if the images are composited or synthesized as they are, an offset will occur in the images in the horizontal position and angle of the traffic lane marking line.
In order to prevent this, it is necessary to correct the horizontal position and angular direction of the traffic lane marking line in the image at the earlier time point by an amount corresponding to the change in the vehicle posture between the present and earlier time points. On this point, the prior art taught by Patent Reference 2 merely detects the amount of vehicle yaw rate change, i.e., presence/absence of vehicle rotation (turning) movement, and corrects the image in correspondence to the amount of turning, if any, so that no correction of the horizontal position is carried out and, therefore, there is a disadvantage in that the traffic lane marking line is not suitably elongated in the composite image. Recognition of the traffic lane marking line by image recognition naturally amounts to recognition of the course of travel (the vehicle's subject or own lane) and since the accuracy with which the direction of the traffic lane marking line is recognized improves with increasing length of the white line components of the traffic lane marking line, it is preferable to composite such that the traffic lane marking line is elongated in appearance.
Therefore, an object of this invention is to overcome the aforesaid drawbacks and provide a traffic lane marking line recognition system for vehicles configured such that the traffic lane marking line is elongated in appearance in the composited image corrected in accordance with the vehicle position (distance between the traffic lane marking line and vehicle) and angle, thereby enabling accurate and unerring recognition of the traffic lane marking line.